1. Field of the Invention
The present invention relates to an electrochemical separation method and apparatus for removing ions, contaminants and impurities from water, fluids, and other aqueous process streams, and for placing the removed ions back into solution during regeneration.
2. Background Art
The separation of ions and impurities from electrolytes has heretofore been generally achieved using a variety of conventional processes including: ion exchange, reverse osmosis, electrodialysis, electrodeposition, and filtering. Other methods have been proposed and address the problems associated with the conventional separation processes. However, these proposed methods have not been completely satisfactory and have not met with universal commercial success or complete acceptance. One such proposed ion separation method is a process for desalting water based on periodic sorption and desorption of ions on the extensive surface of porous carbon electrodes.
The conventional ion exchange process generates large volumes of corrosive secondary wastes that must be treated for disposal through regeneration processes. Existing regeneration processes are typically carried out following the saturation of columns by ions, by pumping regeneration solutions, such as concentrated acids, bases, or salt solutions through the columns. These routine maintenance measures produce significant secondary wastes, as well as periodic interruptions of the deionization process. Secondary wastes resulting from the regeneration of the ion exchangers typically include used anion and cation exchange resins, as well as contaminated acids, bases and/or salt solutions.
In some instances, the secondary radioactive wastes are extremely hazardous and can cause serious environmental concerns. For instance, during plutonium processing, resins and solutions of HNO.sub.3 become contaminated with PuO.sub.2.sup.++ and other radioisotopes. Given the high and increasing cost of disposal of secondary wastes in mined geological repositories, there is tremendous and still unfulfilled need for reducing, and in certain applications, eliminating the volume of secondary wastes.
Another example is the use of the ion exchange process for industrial purposes, such as in the electroplating and metal finishing industries. A major dilemma currently facing the industry relates to the difficulties, cost considerations and the environmental consequences for disposing of the contaminated rinse solution resulting from the electroplating process. A typical treatment method for the contaminated rinse water is the ion exchange process.
Other exemplary processes which further illustrate the problems associated with ion exchange include residential water softening and the treatment of boiler water for nuclear and fossil-fueled power plants. Such water softeners result in a relatively highly concentrated solution of sodium chloride in the drinking water produced by the system. Therefore, additional desalination devices, such as reverse osmosis filters are needed to remove the excess sodium chloride introduced during regeneration.
Therefore, there is still a significant and growing need for a new method and apparatus for deionization and subsequent regeneration, which significantly reduce, if not entirely eliminate secondary wastes in certain applications. The new method and apparatus should enable the separation of any inorganic or organic ion or dipole from any ionically conducting solvent, which could be water, an organic solvent, or an inorganic solvent. For example, it should be possible to use such a process to purify organic solvents, such as propylene carbonate, for use in lithium batteries and other energy storage devices. Furthermore, it should be possible to use such a process to remove organic ions, such as formate or acetate from aqueous streams.
The new method and apparatus should further be adaptable for use in various applications, including without limitation, treatment of boiler water in nuclear and fossil power plants, production of high-purity water for semiconductor processing, removal of toxic and hazardous ions from water for agricultural irrigation, and desalination of sea water.
In the conventional reverse osmosis systems, water is forced through a membrane, which acts as a filter for separating the ions and impurities from electrolytes. Reverse osmosis systems require significant energy to move the water through the membrane. The flux of water through the membrane results in a considerable pressure drop across the membrane. This pressure drop is responsible for most of the energy consumption by the process. The membrane will also degrade with time, requiring the system to be shut down for costly and troublesome maintenance.
Therefore, there is a need for a new method and apparatus for deionization and ion regeneration, which substitute for the reverse osmosis systems, which do not result in a considerable pressure drop, which do not require significant energy expenditure, or interruption of service for replacing the membrane(s).
U.S. Pat. No. 3,883,412 to Jensen describes a method for desalinating water. Another ion separation method relating to a process for desalting water based on periodic sorption and desorption of ions on the extensive surface of porous carbon electrodes is described in the Office of Saline Water Research and Development Progress Report No. 516, March 1970, U.S. Department of the Interior PB 200 056, entitled "The Electrosorb Process for Desalting Water", by Allan M. Johnson et al., ("Department of the Interior Report") and further in an article entitled "Desalting by Means of Porous Carbon Electrodes" by J. Newman et al., in J. Electrochem. Soc.: Electrochemical Technology, March 1971, Pages 510-517, ("Newman Artide"). A comparable process is also described in NTIS research and development progress report No. OSW-PR-188, by Danny D. Caudle et al., "Electrochemical Demineralization of Water with Carbon Electrodes", May, 1966.
The Department of the Interior Report and the Newman Article review the results of an investigation of electrosorption phenomena for desalting with activated carbon electrodes, and discuss the theory of potential modulated ion sorption in terms of a capacitance model. This model desalination system 10, illustrated in FIG. 1, includes a stack of alternating anodes and cathodes which are further shown in FIG. 2, and which are formed from beds of carbon powder or particles in contact with electrically conducting screens (or sieves). Each cell 12 includes a plurality of anode screens 14 interleaved with a plurality of cathode screens 16, such that each anode screen 14 is separated from the adjacent cathode screen 16 by first and second beds 18, 20, respectively, of pretreated carbon powder. These two carbon powder beds 18 and 20 are separated by a separator 21, and form the anode and cathode of the cell 12. In operation raw water is flown along the axial direction of the cells 12, perpendicularly to the surface of the electrode screens 14, 16, to be separated by the system 10 into waste 23 and product 25.
However, this model system 10 suffers from several disadvantages, including:
1. The carbon powder beds 18 and 20 are used as electrodes and are not "immobilized". PA1 2. Raw water must flow axially through these electrode screens 14 and 16, beds of carbon powder 18 and 20, and separators 21, which cause significant pressure drop and large energy consumption. PA1 3. The carbon bed electrodes 18 and 20 are quite thick, and a large potential drop is developed across them, which translates into lower removal efficiency and higher energy consumption during operation. PA1 4. Even though the carbon particles "touch", i.e., adjacent particles are in contact with each other, they are not intimately and entirely electrically connected. Therefore, a substantial electrical resistance is developed, and significantly contributes to the process inefficiency. Energy is wasted and the electrode surface area is not utilized effectively. PA1 5. The carbon beds 18 and 20 have a relatively low specific surface area. PA1 6. The carbon powder bed electrodes 18 and 20 degrade rapidly with cycling, thus requiring continuous maintenance and skilled supervision. PA1 7. The model system 10 is designed for one particular application, sea water desalination, and does not seem to be adaptable for other applications. PA1 1. Unlike conventional processes where water is forced through a membrane by pressure gradient, or where fluid is flown through a packed bed, the CDI separation methods and systems do not require the electrolyte to flow through any porous media such as membranes or packed beds. In the present system, electrolyte flows in open channels formed between two adjacent, planar electrodes, which are geometrically parallel. Consequently, the pressure drop is much lower than conventional processes. The fluid flow can be gravity fed through these open channels, or a pump can be used. PA1 2. The CDI system does not require membranes, which are both troublesome and expensive, which rupture if overpressured, which add to the internal resistance of the capacitive cell, and which further reduce the system energy efficiency. PA1 3. The electrodes in the CDI system are composed of immobilized sorption media, such as monolithic carbon aerogel, which is not subject to entrainment in the flowing fluid stream. Thus, material degradation due to entrainment and erosion is considerably less than in conventional packed carbon columns. PA1 4. The present systems and methods are inherently and greatly energy efficient. For instance, in both evaporation and reverse osmosis processes, water is removed from salt, while in the present systems, salt is removed from water, thus expending less energy. PA1 5. The present systems and methods present superior potential distribution in the thin sheets of carbon aerogel; most of the carbon aerogel is maintained at a potential where electrosorption is very efficient.
Numerous supercapacitors based on various porous carbon electrodes, including carbon aerogel electrodes, have been developed for energy storage applications, and are illustrated in the following:
"Double Layer Electric Capacitor", Nippon Electric Co., Japanese Patent application No. 91-303689, 05211111.
"Electric Double-layer Capacitor", Matsushita Electric Industrial Co., Ltd., Japanese Patent application No. 83-89451, 59214215.
Tabuchi, J., Kibi, Y., Saito, T., Ochi, A., "Electrochemical Properties of Activated Carbon/Carbon Composites for Electric Double-layer Capacitor in New Sealed Rechargeable Batteries and Supercapacitors", presented at the 183rd Electrochemical Society meeting, Honolulu, Hawai, May 16-21, 1993.
"Electrical Double-layer Capacitor, Uses Porous Polarized Electrode Consisting of Carbonized Foamed Phenol Resin", Mitsui Petrochem Ind., Japanese Patent application No. 3,141,629.
Delnick, F. M., Ingersoll, D., Firsich, D., "Double-Layer Capacitance of Carbon Foam Electrodes", SAND-93-2681, Sandia National Laboratory, international seminar report on double layer capacitors and similar energy storage systems, 6-8 Dec. 1993.
Mayer, S. T., Pekala, R. W., Kaschmitter, J. L., "The Aerocapacitor: An Electrochemical Double-layer Energy-Storage Device", J. Electrochemical Society, vol. 140(2) pages 446-451 (February 1993).
U.S. Pat. No. 5,260,855 issued to Kaschmitter et al.
None of these energy storage devices is designed to permit electrolyte flow and most require membranes to physically separate the electrodes. Other electrode materials have been developed for electrolytic cells, e.g. composites of activated carbon powder and an appropriate polymeric binder, as described by Wessling et al., in U.S. Pat. No. 4,806,212. Even though such materials are made from activated carbon powders with very high specific surface areas (600 m.sup.2 /gm), much of the surface is occluded by the binder.
Therefore, there is still a significant unfulfilled need for a new method and apparatus for deionization and regeneration, which, in addition to the ability to significantly reduce, if not completely eliminate, secondary wastes associated with the regeneration of ion exchange columns, do not result in a considerable pressure drop of the flowing process stream, and do not require significant energy expenditure. Furthermore, each electrode used in this apparatus should be made of a structurally stable, porous, monolithic solid. Such monolithic electrodes should not become readily entrained in, or depleted by the stream of fluid to be processed, and should not degrade rapidly with cycling. These electrodes should have a very high specific surface area; they should be relatively thin, require minimal operation energy, and have a high removal efficiency. The new method and apparatus should be highly efficient, and should be adaptable for use in a variety of applications, including, but not limited to sea water desalination.
It would be highly desirable to provide a new class of electrosorption media that may be less susceptible to poisoning and degradation than carbon-based materials, for use in capacitive deionization and regeneration methods and apparatus.
It is likely that continued direct exposure of the electrosorption medium to the electrolytes and chemical regenerants could further degrade the electrodes. Therefore, there is a need for a new separation process that protects the electrosorption medium from the damaging effect of the electrolytes and chemical regenerants, and which does not require the use of chemical regenerants.
Ion exchange chromatography is an analytical method which involves the separation of ions due to the different affinity of the solute ions for the exchanger material. It is a liquid-solid technique in which the ion exchanger represents the solid phase. In ion-exchange separation, the solid phase or column is usually a packed bed of ion exchanger in finely comminuted form; the anion or cation exchanger must be appropriate as the solid phase for the sample of interest. The mobile phase is a solvent such a water with one or more additives such as buffers, neutral salts or organic solvents.
In ion-exchange chromatography the ion-exchanging suppressor column must be periodically regenerated. This is a time-consuming procedure and during the time that the stripper column is being regenerated, the apparatus is not available for use. To minimize the frequency of regeneration, the volume ratios of the suppressor column with respect to the separator column should be kept as low as possible, typically at a ratio of 1:1. This essentially doubles the cost of the ion-exchange materials required.
U.S. Pat. No. 4,672,042 to Ross, Jr. et al., describes an exemplary ion-chromatography system. Two separate exchange columns, anionic and cationic, are required, which increases the cost of the chromatograph, and essentially doubles the cost of the ion exchange or packing material required. Solid ion exchange column packings are used, which limit the applications of the chromatograph and require a significantly higher energy to operate compared to a single hollow column.
Several attempts have been made to select an appropriate ion-exchange composition for the solid phase, e.g. U.S. Pat. No. 5,324,752 to Barretto et al.; U.S. Pat. No. 4,675,385 to Herring; U.S. Pat. No. 5,294,336 to Mizuno et al.; U.S. Pat. No. 4,859,342 to Shirasawa; U.S. Pat. Nos. 4,952,321 and 4,959,153 to Bradshaw et al.
However, these devices and methods are specifically designed for particular applications, and a single chromatograph cannot be used universally in various applications. Additionally, these conventional devices require the use of multiple columns.
Therefore, there is still a significant unfulfilled need for a new and versatile chromatograph and method of operation, which uses a single column for simultaneous anionic and cationic types chromatography. This new chromatograph should control the elution time of the species being analyzed, should have reduced overall cost of manufacture, operation and maintenance, should use electrical rather than chemical regeneration and should use the same column (or stack of cells) for both anions and cations.